Fuel cut - what does the DME do?
05-27-2005, 06:03 PM
#1
Fuel cut - what does the DME do?
First off, I'm talking about the DME's FUEL CUT, not the KLR's boost cut. Second, I'm not asking when fuel cut happens. I know it comes on from sustained high airflow for a given period of time.
Alright, the question: What action does the DME electrically perform when it decides to enable fuel cut?
The reason I ask is because I've datalogged a very sharp dive in the AFM signal voltage when fuel cut occured. It represented a DRASTIC reduction in airflow, more than I expected to see.
Possibility #1 - What if the DME cut power to the AFM to trigger fuel cut? It'd work because:
Too much airflow -> DME decides to cut fuel -> AFM gets it's power shut off -> airflow signal from AFM indicates zero airflow (0.0 volts) -> DME uses 0.0 volts to calculate fuel injector cycling -> DME doesn't fire the injectors.
Possibility #2 - The DME simply shuts off the injectors directly and the airflow through the AFM takes a dive as a result, but I have to say that's one hell of an airflow change.
Anyone ever bother to check into how the DME performs fuel cut? Any thoughts?
Alright, the question: What action does the DME electrically perform when it decides to enable fuel cut?
The reason I ask is because I've datalogged a very sharp dive in the AFM signal voltage when fuel cut occured. It represented a DRASTIC reduction in airflow, more than I expected to see.
Possibility #1 - What if the DME cut power to the AFM to trigger fuel cut? It'd work because:
Too much airflow -> DME decides to cut fuel -> AFM gets it's power shut off -> airflow signal from AFM indicates zero airflow (0.0 volts) -> DME uses 0.0 volts to calculate fuel injector cycling -> DME doesn't fire the injectors.
Possibility #2 - The DME simply shuts off the injectors directly and the airflow through the AFM takes a dive as a result, but I have to say that's one hell of an airflow change.
Anyone ever bother to check into how the DME performs fuel cut? Any thoughts?
05-28-2005, 03:06 AM
#2
Instructor
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Don't know the answer to this, but I have occasionally found the same drastic voltage reduction in my datalogs when the fuel cut has happened - this is with the AFM Link fitted.
Regards
Graham
'88 Turbo S
Regards
Graham
'88 Turbo S
05-28-2005, 08:20 AM
#3
Burning Brakes
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Interesting question! I guess the answer is linked to how the DME first determines that the engine is overboosting, right? Purely guessing, if the DME reads excessive airflow and shuts the power to the AFM then it can use an existing routine to control the injectors. If I were programming things that would seem to an elegant solution as opposed to teaching it that there are two different reasons for shutting down the injectors.
05-28-2005, 09:20 AM
#4
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From what I see in the DME circuit diagram the AFM power is not controlled by the microcontroller.
Although I've not yet verified it in the DME code I'm quite sure that the DME just don't open the injectors when it wants to cut fuel.
Tomas
Although I've not yet verified it in the DME code I'm quite sure that the DME just don't open the injectors when it wants to cut fuel.
Tomas
05-28-2005, 10:58 AM
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Originally Posted by special tool
Thomas - where you been lately? Making meatballs or something? 
Well back to the garage!
